The present invention relates to integrated circuits, and more particularly, to a pilot-sensor style power output of an integrated circuit.
Often, a high-power output from an integrated circuit (IC) must have a fail-safe current limiting function built-in to the device to prevent damage either to the IC or the load which it must drive. There are two conventional methods to sense the current through the IC driver output. The simpler method uses an explicit or parasitic resistance in the current path. This resistance, which must be predictable, causes a voltage drop which can be compared to a reference voltage. This voltage drop across the sense resistance is usually undesirable because it reduces the output voltage swing and generates on-chip power waste.
The second method introduces no series voltage drop in the output path, but instead builds a parallel, sense field effect transistor (FET), also known as a pilot device, which runs at a known reference current level. This pilot device is matched exactly to the driver (i.e. the power FET), except the pilot device is scaled down significantly in size and in current from the driver. In this way, the pilot and the driver will be running the same current densities at the trip-point for the limiter (the maximum driver current).
U.S. Pat. No. 4,553,084 to Wrathall describes a current sensing circuit with a pilot sense transistor having a sensing resistor in its source leg. The pilot sense transistor is a portion of a large switching transistor and so variations in the large switching transistor can be accurately tracked by the pilot sense transistor. An operational amplifier monitors the signal provided by the sensing resistor for providing feedback information to a driver circuit. Wrathall describes another apparatus for sensing load current in U.S. Pat. No. 4,820,968 which uses a sense resistor in the leg of a current mirroring transistor to convert a mirrored current into a voltage. This voltage is then compared to a reference voltage generated by a reference current which is a equal to a portion of the load current.
The drawbacks to the pilot sensor method are that the pilot reference current is waste-current, and that the scaled-match from pilot to driver becomes more difficult and improbable if the ratio is very large (e.g. 50,000:1). There is, in fact, a trade-off between the waste-current used and increasingly large size ratio. If the IC is to have low power dissipation and accepts the very large ratio of sizes between the pilot and driver FETS, either the current limiter tolerances must be made very loose in the specification, or the part will have to be trimmed to compensate for the inaccuracies of fabrication. Trimming the part adds cost to the part due to increased test time and chip area consumed by the trim circuit.
In view of the foregoing background, it is therefore an object of the invention to provide a circuit and associated method for sensing current in a power output of an integrated circuit with low power dissipation, reduced need to trim the circuit, and reduced sensitivity to variations in fabrication.
This and other objects, features and advantages in accordance with the present invention are provided by an integrated circuit including a power driving device, and a pilot device for sensing current through the power driving device. The pilot device includes a composite pilot having a plurality of series connected transistors operative at least while the power driving device is in a linear mode, and a secondary pilot operative while the power driving device is operating in a saturation mode. Also, a control circuit activates the secondary pilot.
Preferably, the power driving device comprises a power field effect transistor (FET), and the composite pilot and the secondary pilot comprise sense FETs scaled and matched to the power FET. The pilot device generates a reference voltage and an amplifier may compare the reference voltage with a source voltage of the power FET. Such an amplifier controls gate voltages of the power FET and the sense FETs.
The pilot device also includes a first current source connected to the composite pilot, and a second current source connected to the secondary pilot. The control circuit controls the second current source to activate the secondary pilot. The control circuit may comprise an over-current detector for detecting whether the power driving device is operating in the saturation mode. Furthermore, the composite pilot may comprise at least twenty series connected sense FETs.
Objects, features and advantages in accordance with the present invention are also provided by a method of regulating current in an integrated circuit including a power driving device. The method includes sensing current through the power driving device and generating a reference voltage with a first pilot at least while the power driving device is in a linear mode, detecting whether the power driving device is in a saturation mode, and sensing current through the power driving device and generating the reference voltage with a second pilot while the power driving device is in the saturation mode. The method may also include comparing the reference voltage with a source voltage of the power FET and controlling gate voltages of the power FET and the sense FETs based upon the comparison of the reference voltage with the source voltage of the power FET.